1. Field of the Invention
The present invention relates to stepping motor type indicators for indicating various types of varying measurements, such as the running speed or engine speed of a vehicle, by the pointing direction or angle of a pointer which swings over a dial. More particularly, the invention pertains to a structure for supporting the pointer.
2. Description of the Related Art
A PM type stepping motor is a device widely used as an actuator in conventional office automation equipment, home electronic appliances and motor vehicles, for instance. The PM-type stepping motor allows easy control of forward and reverse running operation and its physical size can be easily reduced depending on required driving torque. For this reason, the PM-type stepping motor attracts growing attention in recent years as a prime mover for instruments as a substitute for air-core type movements of cross coil instruments.
As disclosed in JP-A-6-153484, for instance, a stepping motor of this type mostly comprises a rotor having an output shaft and constructed essentially of a permanent magnet and a stator formed of a yoke having an array of pole teeth arranged face to face with a curved outer surface of the rotor and exciting coils for magnetizing the pole teeth, wherein driving signals having controlled phase differences are fed into the individual exciting coils of the stator to magnetize the pole teeth and thereby produce a rotating magnetic field, which causes the rotor to rotate.
With a pointer fixed to the output shaft and a dial plate fixed between the pointer and the stepping motor, the driving signals fed into the exciting coils are controlled in accordance with a measurement value (e.g., the varying running speed or engine speed of a vehicle), whereby the motor can be used as a prime mover of an indicator.
In a case where the stepping motor is used as a prime mover of an indicator as described above, it is necessary to hold the pointer stationary at a specified position over the dial plate and suppress undesirable movements of the pointer during a period when no driving voltage signals are applied to the exciting coils. An effective means for achieving this is rotor (pointer) arresting means which mechanically stops pointer movements.
In the stepping motor, however, complicated magnetic circuits are formed between the rotor and the stator including the pole tooth array and the magnetic circuits produce magnetic forces which act on the rotor even under a non-excited condition, although the magnetic forces are considerably small. Therefore, with the provision of the rotor arresting means alone, the pointer is apt to depart from the position where it was set by the rotor arresting means. In this circumstance, there has been the need for an effective means of positively holding the pointer at a specified position.
Taking into consideration the aforementioned problem of the prior art, the applicants of the present invention previously proposed in JP-A-9-184740 an arrangement for holding a pointer at a specified position by using magnetic forces exerted on a rotor under the non-excited condition. This previous arrangement was devised in the light of the fact that the magnetic forces exerted by a yoke on the rotor under the non-excited condition produce a plurality of stable points which attract and hold the pointer (rotor). More specifically, a stopping position of the pointer determined by rotor arresting means is set at a point located on a decreasing side of a middle point between two adjacent stable points existing along increasing and decreasing directions of the pointer in the previous arrangement.
However, the strength of holding forces of such stable points could differ from one product to another due to assembly errors or dimensional errors of constituent components, or the stable points which could force or attract the rotor to specific positions might not be positively formed in certain cases. Even when a stopping position of the pointer is set in consideration of the location of the stable points as described above, it might not be possible to positively hold the pointer at the set stopping position if the stable points formed are ambiguous and the holding forces of the stable points are too small. It is understood from the above discussion that there are limitations in the aforementioned previous arrangement as the means of holding the pointer at a specified position under non-excited conditions and, therefore, there has been a continuing need for improvement.
It is an object of the invention to provide a stepping motor type indicator whose stepping motor forms unambiguous stable points which produce a sufficiently strong holding force in a non-excited condition, making it possible to suppress undesirable movements of a pointer.